Liquid development system

ABSTRACT

An electrophotographic printing machine in which an electrostatic latent image recorded on a photoconductive surface is developed with a liquid developer material comprising at least a liquid carrier having marking particles therein. The liquid developer material is furnished to the electrostatic latent image recorded on the photoconductive surface in a development zone to develop the latent image. Marking particles are substantially uniformly dispersed in the liquid carrier of the liquid developer material at the entrance to the development zone so as deflocculate the marking particles.

This invention relates generally to an electrophotographic printingmachine, and more particularly concerns an apparatus for developing anelectrostatic latent image recorded on a photoconductive surface with aliquid developer material comprising a liquid carrier having markingparticles dispersed therein.

A typical electrophotographic printing machine employs a photoconductivemember that is charged to a substantially uniform potential so as tosensitize the surface thereof. The charged portion of thephotoconductive member is exposed to a light image of an originaldocument being reproduced. Exposure of the charged photoconductivemember selectively dissipates the charge, in the irradiated areas, torecord an electrostatic latent image on the photoconductive membercorresponding to the informational areas contained within the originaldocument. After the electrostatic latent image is recorded on thephotoconductive member, the latent image is developed by bringing adeveloper material into contact therewith. A dry developer materialcomprising carrier granules having toner particles adhering thereto isbrought into contact with the latent image. The latent image attractsthe toner particles from the carrier granules to form a toner powderimage on the photoconductive surface. The toner powder image is thentransferred to the copy sheet and, thereafter, permanently fusedthereto.

Development of an electrostatic latent image may also be achieved with aliquid developer material rather than a dry developer material. In aliquid development system, an insulating liquid carrier having finelydivided marking particles dispersed therein contacts the photoconductivesurface. Under the influence of the electrical field associated with theelectrostatic latent image, the marking particles are attracted to thephotoconductive surface to form a visible image. It has been found thatwhen a liquid developer material is employed, the midtone solid areas ofthe developed image frequently exhibit a mottled appearance. It isbelieved that one of the underlying mechanisms causing the mottledappearance in the mid-tone solid area is due to flocculation, i.e. theformation of agglomerates, of marking particles in the liquid carrier.Thus, if the agglomerates of marking particles in the liquid carrier arebroken up, i.e., deflocculated, a significant improvement in solid areadevelopment occurs. Various types of development systems have beenemployed with liquid development materials. The following disclosuresappear to be relevant:

U.S. Pat. No. 3,576,623

Patentee: Snelling

Issued: Apr. 27, 1971

U.S. Pat. No. 3,965,861

Patentee: Fukushima et al.

Issued: June 29, 1976

U.S. Pat. No. 4,073,266

Patentee: Arneth et al.

Issued: Feb. 14, 1978

U.S. Pat. No. 4,077,712

Patentee: Imaizumi et al.

Issued: Mar. 7, 1978

The relevant portions of the foregoing patent may be briefly summarizedas follows:

Snelling describes a development system employing a coronode immersed ina liquid developer. The coronode is positioned in the development zoneto control development.

Fukushima et al. discloses a development roll which serves as adeveloping electrode and transports a liquid developer material intocontact with the electrostatic latent image recorded on an image bearingmaterial. A member is interposed between a pair of developed rollers andfurnishes developer material which runs over the sides thereof betweenadjacent developer rollers. The developer material is supplied onto theimage bearing surface in a laminar pattern. The Reynolds Number ismaintained below 2000 to maintain laminar flow and avoid turbulence.

Arneth et al. describes an apparatus for developing an electrostaticlatent image recorded on a copying material. A voltage having the samepolarity as the surface charge of the electrostatic latent image isapplied to a distribution roller to eliminate thin white lines on theimage.

Imaizumi et al. discloses a developing device employing a liquiddeveloping solution for use in an electrophotographic printing machine.A plurality of electrode rollers are immersed in the developing solutionto provide efficient circulation of the developing solution and rapidand uniform dispersion of toner particles in the developing solution.

In accordance with one aspect of the features of the present invention,there is provided an apparatus for developing an electrostatic latentimage with a liquid developer material comprising at least a liquidcarrier having marking particles dispersed therein. Means are providedfor furnishing the liquid developer material to the electrostatic latentimage in a development zone to develop the electrostatic latent image.Means disperse the marking particles substantially uniformly in theliquid carrier at the entrance to the development zone so as todeflocculate the marking particles therein.

Pursuant to another aspect of the features of the present invention,there is provided an electrophotographic printing machine of the typehaving an electrostatic latent image recorded on a photoconductivesurface developed with a liquid developer material comprising at least aliquid carrier having marking particles dispersed therein. Means areprovided for furnishing the liquid developer material to theelectrostatic latent image recorded on the photoconductive surface in adevelopment zone to develop the electrostatic latent image. Meansdisperse the marking particles substantially uniformly in the liquidcarrier at the entrance to the development zone so as to deflocculatethe marking particles therein.

Other aspects of the present invention will become apparent as thefollowing description proceeds and upon reference to the drawings, inwhich:

FIG. 1 is a schematic elevational view depicting an electrophotographicprinting machine incorporating the features of the present inventiontherein; and

FIG. 2 is an elevational view showing the development apparatus used inthe FIG. 1 printing machine.

While the present invention will hereinafter be described in connectionwith a preferred embodiment thereof, it will be understood that it isnot intended to limit the invention to that embodiment. On the contrary,it is intended to cover all alternatives, modifications and equivalentsas may be included within the spirit and scope of the invention asdefined by the appended claims.

For a general understanding of the features of the present invention,reference is made to the drawings. In the drawings, like referencenumerals have been used throughout to designate identical elements. FIG.1 is a schematic elevational view illustrating an electrophotographicprinting machine incorporating the features of the present inventiontherein. It is will become apparent from the following discussion thatthe apparatus of the present invention is equally well suited for use ina wide variety of printing machines and is not necessarily limited inits application to the particular embodiment shown herein.

Turning now to FIG. 1, the printing machine employs a belt 10 having aphotoconductive surface deposited on a conductive substrate. Preferably,the photoconductive surface is made from a selenium alloy with theconductive substrate being preferably made from an aluminum alloy whichis electrically grounded. Belt 10 advances successive portions of thephotoconductive surface sequentially through the various processingstations disposed about the path of movement thereof. The supportassembly for belt 10 includes three rollers 12, 14 and 16 located withparallel axes approximately at the apexes of a triangle. Roller 12 isrotatably driven by a suitable motor and drive (not shown) so as torotate and advance belt 10 in the direction of arrow 18.

Initially, belt 10 passes through charging station A. At chargingstation A, a corona generating device 20 charges the photoconductivesurface of belt 10 to a relatively high, substantially uniformpotential.

After the photoconductive surface of belt 10 is charged, the chargedportion thereof is advanced to exposure station B. At exposure stationB, an original document 22 is placed upon a transparent support platen24. An illumination assembly, indicated generally by the referencenumeral 26, illuminates the original document 22 on platen 24 toproduced image rays corresponding to the informational areas of theoriginal document. The image rays are projected by means of an opticalsystem onto the charged portion of the photoconductive surface. Thelight image dissipates the charge in selected areas to record anelectrostatic latent image on the photoconductive surface whichcorresponds to the informational areas contained within originaldocument 22.

After the electrostatic latent image has been recorded on thephotoconductive surface of belt 10, belt 10 advances the electrostaticlatent image to development station C. At development station C, aroller 28, rotating in the direction of arrow 30, advances a liquiddeveloper material 32 comprising marking particles, i.e. tonerparticles, in a insulating liquid carrier from the chamber of housing 34to development zone 36. An electrode 38 positioned before the entranceto development zone 36 is electrically biased to generate an AC fieldjust prior to the entrance to development zone 36 so as to disperse themarking particles substantially uniformly throughout the liquid carrierin this region. This causes deflocculation of the marking particles.inthe liquid carrier. The detailed arrangement of development station Cwill be described hereinafter with reference to FIG. 2. The markingparticles, disseminated through the liquid carrier, pass byelectrophoresis to the electrostatic latent image. The charge of themarking particles is opposite in polarity to the charge on thephotoconductive surface. For example, if the photoconductive surface ismade from a selenium alloy, the corona charge will be positive and thetoner particles will be negatively charged. Alternatively, if thephotoconductive surface is made from a cadmium sulphide material, thecharge will be negative and the toner particle will have a positivecharge. A suitable developer material is described in U.S. Pat. No.4,582,774 issued to Landa in 1986, the relative portions thereof beinghereby incorporated into the present application. By way of example, theinsulating carrier liquid may be a hydrocarbon liquid although otherinsulating liquids may also be employed. A suitable hydrocarbon liquidis an Isopar which is a trademark of the Exxon Corporation. These arebranched, chained aliphatic hydrocarbon liquids (largely decane). Thetoner particles comprise a binder and a pigment. The pigment may becarbon black. However, one skilled in the art will appreciate that anysuitable liquid development material may be employed.

After the electrostatic latent image is developed, belt 10 advances thedeveloped image to transfer station D. At transfer station D, a sheet ofsupport material 40 is advanced from stack 42 by a sheet transportmechanism, indicated generally by the reference numeral 44. Transferstation D includes a corona generating device 46 which sprays ions ontothe backside of the sheet of support material 40. This attracts thedeveloped image from the photoconductive surface of belt 10 to copysheet 40. After transfer, conveyor belt 48 moves the copy sheet tofusing station E.

Fusing station E includes a fuser assembly, indicated generally by thereference numeral 50, which permanently fuses the developed image to thecopy sheet. Fuser assembly 50 includes a heated fuser roll 42 andback-up or pressure roll 54 resiliently urged into engagement therewithto form a nip through which the copy sheet passes. After fusing, thefinished copy sheet is discharged to output tray 6 for removal therefromby the machine operator.

With continued reference to FIG. 1, after a developed image istransferred to the copy sheet, residual liquid developer materialremains adhering to the photoconductive surface of belt 10. A cleaningroller 58 formed of any appropriate synthetic resin, is driven in adirection opposite to the direction of movement of belt 10 to scrub thephotoconductive surface clean. To assist in this action, developingliquid may be fed through pipe 60 to the surface of cleaning roller 8. Awiper blade 62 completes the cleaning of the photoconductive surface.Any residual charge left on the photoconductive surface is extinguishedby flooding the photoconductive surface with light from lamps 64.

Referring now to FIG. 2, there is shown the detailed structure ofdevelopment station C. As shown thereat, development station C includesa developer roller 28 rotating in the direction of arrow 30 so as topass through developer material 32 located in the chamber of housing 34.Developer roller 28 transports developer material into development zone36 so that the electrostatic latent image recorded on thephotoconductive surface of belt 10 attracts the marking particlesthereto to form a visible image thereon. Developer roller 28 may eithercontact or be slightly apart from the photoconductive surface. Developerroller 28 is driven in the direction of arrow 30 by a suitable drivingdevice which is not illustrated. Roller 28 is conductive, being madefrom metal or any other appropriate material and serves as a developingelectrode. Thus, roller 28 is electrically biased to a suitablepotential and magnitude. Development electrode 38 is located before theentrance to development zone 36 and spaced from developer roller 28.Electrode 38 generates a pulsed electrical field so as to move themarking particles in the liquid carrier by electrophoresis tosubstantially uniformly disperse the marking particles in the liquidcarrier prior to entering development zone 36. This is achieved byelectrically biasing electrode 38 with a pulsed generator, i.e. anysuitable pulse may be employed. However, it is preferable to utilize anAC generator. Thus, an alternating electrical field is generated whichmoves the marking particles in the liquid carrier to substantiallyuniformly disperse the marking particles in the liquid carrier, therebydeflocculating the marking particles in the liquid developer materialjust prior to entering development zone 36. Electrode 38 is electricallybiased by an alternately current voltage source 66 connected thereto.

One skilled in the art will appreciate that any suitable technique tocreate motion of the marking particles in the liquid carrier may beemployed. For example, an acoustic generator may be used to transmitsound waves through the liquid developer material in the region prior tothe entrance to development zone 36. The frequency of these sound wavesis such that the liquid developer material is agitated and becomesturbulent. Still another approach is to employ mechanical means such asa stirrer or mixing device which agitates and mixes the liquid developermaterial just prior to the entrance to the development zone so as toonce again create turbulence therein. In any case, any suitabletechnique may be utilized so long as the marking particles are inducedto disperse substantially uniformly in the liquid carrier. In thismanner, the agglomerates of marking particles are broken up, ordeflocculated.

Deflocculation is achieved by dispersing the marking particlessubstantially uniformly throughout the liquid carrier by the use ofeither an AC electrical field, which causes marking particle motionthrough the liquid carrier by electrophoresis, or by mixing the liquiddeveloper material, which can be caused by mechanical or acousticaldevices. Accordingly, marking particle deflocculation can be achieved bymixing the liquid developer material, i.e. causing turbulence therein,or by moving the marking particles through the liquid carrier byelectrophoresis. Generally, The charge of each marking particle and theStoke's radius of each marking particle are different so that eachmarking particle migrates a different distance when the electrical fieldis applied thereon resulting in deflocculation of the marking particlesin the liquid carrier.

In recapitulation, it has been found that by inducing dispersion of themarking particles in the liquid developer material just prior toentering the development zone, deflocculation occurs. When theagglomerates of marking particles are broken up and substantiallyuniformly dispersed throughout the liquid carrier, mottling issignificantly reduced in the resultant copy. Substantially uniformdispersion of the marking particles in the liquid carrier of thedeveloper material may be induced by electrical, acoustical ormechanical devices. It is, thus, clear that the development system ofthe present invention minimizes the formation of mottled regions insolid areas of the copy by deflocculating the marking particles justprior to entrance into the development zone resulting in significantlyimproved copy quality.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, an apparatus for developing an electrostaticlatent image with a liquid developer material. This apparatus fullysatisfies the aims and advantages hereinbefore set forth. While thisinvention has been described in conjunction with a specific embodimentthereof, it is is evident that many alternatives, modifications, andvariations will be apparent to those skilled in the art. Accordingly, itis intended to embrace all such alternatives, modifications andvariations that fall within the spirit and broad scope of the appendedclaims.

I claim:
 1. An apparatus for developing an electrostatic latent imagewith a liquid developer material comprising at least a liquid carrierhaving marking particles therein, including:means for furnishing theliquid developer material to the electrostatic latent image in adevelopment zone to develop the electrostatic latent image; and meansfor dispersing the marking particles substantially uniformly in theliquid carrier of the liquid developer material at the entrance to thedevelopment zone so as to deflocculate the marking particles.
 2. Anapparatus for developing an electrostatic latent image with a liquiddeveloper material comprising at least a liquid carrier having markingparticles therein, including:means for furnishing the liquid developermaterial to the electrostatic latent image in a development zone todevelop the electrostatic latent image; and means for dispersing themarking particles substantially uniformly in the liquid carrier of theliquid developer material at the entrance to the development zone so asto deflocculate the marking particles, said dispersing means comprisesmeans for generating a pulsed electrical field in the developer materialat the entrance to the development zone to induce movement of themarking particles in the liquid carrier of the liquid developermaterial.
 3. An apparatus according to claim 2, wherein said generatingmeans includes;an electrode positioned at the entrance to thedevelopment zone; and means for applying a pulsed voltage to saidelectrode to generate an pulsed electrical field in the developermaterial at the entrance to the development zone.
 4. An apparatusaccording to claim 3, wherein said applying means generates analternating voltage.
 5. An apparatus according to claim 1, wherein saiddispersing means includes means for mechanically mixing the developermaterial at the entrance to the development zone.
 6. An apparatus fordeveloping an electrostatic latent image with a liquid developermaterial comprising at least a liquid carrier having marking particlestherein, including:means for furnishing the liquid developer material tothe electrostatic latent image in a development zone to develop theelectrostatic latent image; and means for dispersing the markingparticles substantially uniformly in the liquid carrier of the liquiddeveloper material at the entrance to the development zone so as todeflocculate the marking particles, said dispersing means comprisesmeans for acoustically vibrating the developer material at the entranceto the development zone.
 7. An electrophotographic printing machine ofthe type having an electrostatic latent image recorded on aphotoconductive surface developed with a liquid developer materialcomprising at least a liquid carrier having marking particles therein,wherein the improvement includes:means for furnishing the liquiddeveloper material to the electrostatic latent image recorded on thephotoconductive surface in a development zone to develop theelectrostatic latent image; and means for dispersing the markingparticles substantially uniformly in the liquid carrier of the liquiddeveloper material at the entrance to the development zone so as todeflocculate the marking particles.
 8. An electrophotographic printingmachine of the type having an electrostatic latent image recorded on aphotoconductive surface developed with a liquid developer materialcomprising at least a liquid carrier having marking particles therein,wherein the improvement includes:means for furnishing the liquiddeveloper material to the electrostatic latent image recorded on thephotoconductive surface in a development zone to develop theelectrostatic latent image; and means for dispersing the markingparticles substantially uniformly in the liquid carrier of the liquiddeveloper material at the entrance to the development zone so as todeflocculate the marking particles, said dispersing means comprisesmeans for generating a pulsed electrical field in the developer materialat the entrance to the development zone.
 9. A printing machine accordingto claim 8, wherein said generating means includes;an electrodepositioned at the entrance to the development zone; and means forapplying a pulsed voltage to said electrode to generate a pulsedelectrical field in the developer material at the entrance to thedevelopment zone.
 10. A printing machine according to claim 9, whereinsaid applying means generates an alternating voltage.
 11. A printingmachine according to claim 7, wherein said dispersing means includesmeans for mechanically mixing the developer material at the entrance tothe development zone.
 12. An electrophotographic printing machine of thetype having an electrostatic latent image recorded on a photoconductivesurface developed with a liquid developer material comprising at least aliquid carrier having marking particles therein, wherein the improvementincludes:means for furnishing the liquid developer material to theelectrostatic latent image recorded on the photoconductive surface in adevelopment zone to develop the electrostatic latent image; and meansfor dispersing the marking particles substantially uniformly in theliquid carrier of the liquid developer material at the entrance to thedevelopment zone so as to deflocculate the marking particles, saiddispersing means comprises means for acoustically vibrating thedeveloper material at the entrance to the development zone.